Add elapsed month and quarters, stored as Integers

Elapsed month and quarters are useful in a lot of cases. An example is to use the `roll` option with respect to  1 month (since one cannot use `roll = month(1)`). 
yearmon and yearqtr exist in zoo but they are stored as elapsed year + 1/12 or 1/4. 
I wrote these integer classes for personal use. Discovering IDate and ITime, it makes sense to add them here.